Conrad Pitts, a lawyer for Sunshine Mills, said 80 percent of the tainted biscuits were sold by Wal-Mart, under the Ol’ Roy brand. Mr. Pitts said that the company had produced about 24 truckloads of biscuits with the contaminated gluten, and that the majority of the product was large biscuits. He said wheat gluten accounted for less than 1 percent of the total weight of the biscuits.

Menu Foods, which last month recalled more than 90 brands of its “cuts and gravy” pet food, said yesterday that it had extended the period of time covered by its recall to include food made after Nov. 8, 2006. The company, based in Ontario, initially recalled only food made from Dec. 3, 2006, to March 6, 2007.

Menu Foods said it acted after a supplier, ChemNutra of Las Vegas, recalled all wheat gluten it had imported from the Xuzhou Anying Biologic Technology Development Company of Wangdien, China. ChemNutra said Wednesday that the F.D.A. had found melamine in the gluten. The agency said it was now testing all wheat gluten from China.

The Chinese government said yesterday that no wheat gluten had been exported to the United States or Canada. Xuzhou Anying denied it had ever shipped wheat gluten to either country.

“We are a trading company and don’t manufacture the product,” added Mao Lijun, the company’s general manager. Michael Rogers, director of the Division of Field Investigations for the F.D.A., said records showed that the tainted gluten came from China.

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Mr. Rogers and Dr. Sundlof said the gluten did not enter the human food supply. The agency said that it was still investigating how the melamine got into the gluten, and that it had notified all companies that had received it.

Though melamine has been found in the food and in the urine and kidneys of pets that have eaten the food, officials and scientists are not sure whether the chemical actually caused pets to get sick.

Melamine, which is also used as a slow-release fertilizer, is generally not known to be toxic. Some theories are that it might act as a marker for another unknown toxin that causes renal failure in pets, Dr. Sundlof said, or that cats and dogs are extremely sensitive to melamine.

“We still have a lot of work to do to understand why melamine is involved, as it is a relatively nontoxic substance,” Dr. Sundlof said. “We are relatively certain that there is a connection here someplace.”

The F.D.A. said it had received more than 12,000 complaints about pet food since the recall, as many as it usually gets in a two-year period on all topics combined. It has confirmed 16 deaths.

“We have no good information what that final number might be,” Dr. Sundlof said. “It will take a while for us to get there.”

The College of Veterinary Medicine at Iowa State University is investigating 43 suspected cases of pets that died from eating tainted food; the deaths of 18 are consistent with ingestion of a toxin, said Patrick Halbur, executive director of the Veterinary Diagnostic Lab at Iowa State.

Dr. Halbur and Grant Maxie of the University of Guelph in Ontario, which is also investigating the cause of the illnesses and deaths, said it would probably take months to determine what made the pets sick.

In Chicago, Senator Richard J. Durbin, Democrat of Illinois, called for a hearing to question F.D.A. officials. He also called for standardized federal regulations and inspection requirements for pet food.

David Barboza contributed reporting from Shanghai, and Libby Sander from Chicago.

A version of this article appears in print on , on Page A15 of the New York edition with the headline: 22 Brands of Dog Biscuits Are Added to Pet Food Recall. Order Reprints|Today's Paper|Subscribe